The present invention relates to a method and an apparatus for measuring the thickness of a coating provided around a cylindrical object such as an optical fiber optically.
Quartz based fiber optics have a tendency to break if they are left to stand under tensile stress for a prolonged time. This phenomenon is commonly referred to as "fatigue". One of the methods for dealing with the fatigue problem is to prevent the growth of surface flaws by insuring that the surface energy of a quartz glass fiber is always held at high level. A specific way to perform this method is shown in FIG. 1. In FIG. 1, an optical fiber 30 including a doped silica core 31 and a silica cladding 32 is coated with a less water-permeable pyrolytic carbon film 33 (which is hereinafter referred to as "a carbon coating") by a CVD technique so that water will not be adsorbed on the surface of the fiber. Films such as this carbon coating that are closely adherent to a substrate (in this case, optical fiber 30) and which are less permeable to gases are commonly referred to as "hermetic coatings".
The optical fiber having the carbon coating film 33 benefits from the preferred property of low water permeation and this property is known to vary greatly with the thickness of the carbon coating. If the thickness of the coating is increased in order to reduce the permeation of H.sub.2 and water as much as possible, an increasing amount of by-products will be deposited on the inside surface of the reaction tube during fiber production and the resulting contact of the deposit with the bare fiber causes its initial strength to decrease, thereby making it impossible to continue the fiber production for a long time.
Hence, the thickness of carbon film that is sufficient to prevent water permeation and which will not cause any adverse effects of by-products should be held within certain limits and it is necessary to control the film thickness during fiber production. To this end, the first thing to do is to measure the correct thickness of the carbon film.
Measuring film thicknesses in the range of ca. 200-1,000 .ANG. which are generally considered to be preferable for practical carbon coatings requires complex analytical techniques such as FE-SEM (Field Emission-Secondary Electron Microscope) and AES (Auger electron spectroscopy) in which an object to be measured is irradiated by an electron beam and an energy of Auger electrons generated from the surface of the object is measured to detect the composition of the object and, hence, the development of a means that allows the film thickness of carbon coatings to be easily checked during fiber production has been desired.
Using the correlation between the thickness of a film and its electric resistance, the present inventors proposed that the thickness of carbon coatings not thicker than 0.1 .mu.m be determined nondestructively by measuring their electric resistance.
A circuit for implementing this method is shown in FIG. 2(a), in which the electric resistance of an optical fiber 41 having a pyrolytic carbon coating is measured with guides 42 and 43 made of a metal such as phosphor bronze being used as electric contacts. Shown by 44 is not an electric contact but a mere guide. Another circuit is shown in FIG. 2(b), in which the measurement of electric resistance is conducted with melts of a metal such as mercury 45 in dies 46 and 47 through which pass the optical fiber 41 being used as electric contacts.
In the methods described above, the optical fiber 41 to be measured has to be placed in contact with guides 42 and 43 or molten metal 45 and there is high likelihood that the mechanical strength of the fiber will deteriorate as a result of this fiber contact with other objects. Thus, it has been desired to develop a nondestructive and non-contact method for measuring the thickness of coating films.